Annual report pursuant to Section 13 and 15(d)

Industry, Segment and Geographic Financial Information - Concentration (Details)

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Industry, Segment and Geographic Financial Information - Concentration (Details) - customer
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Top five customers      
Significant Customers and Vendors      
Number of customers 5 5 5
Net sales | Customer concentration risk      
Significant Customers and Vendors      
Number of customers 2    
Net sales | SHI | Customer concentration risk      
Significant Customers and Vendors      
Percentage of concentration risk 16.60% 20.10% 16.30%
Net sales | CDW | Customer concentration risk      
Significant Customers and Vendors      
Percentage of concentration risk 25.60% 18.00% 17.30%
Net sales | Top five customers      
Significant Customers and Vendors      
Percentage of concentration risk 55.00% 50.00% 46.00%
Net accounts receivable | SHI | Customer concentration risk      
Significant Customers and Vendors      
Percentage of concentration risk 15.00% 14.90%  
Net accounts receivable | CDW | Customer concentration risk      
Significant Customers and Vendors      
Percentage of concentration risk 35.60% 28.20%  
Purchases | Vendor concentration risk | SolarWinds      
Significant Customers and Vendors      
Percentage of concentration risk 15.30% 14.70% 10.80%
Purchases | Vendor concentration risk | Sophos      
Significant Customers and Vendors      
Percentage of concentration risk 23.90% 26.40% 23.10%